# Sizing of generator

#### engrkira

##### Member
Good day.

I would like to ask on how to size a generator set with given loads.

I have 2 heaters rated as 19 kW and a 30 kW motor. The genset will run 16 hours per day so the generator to be used should be Prime Power.

Appreciated if you can teach it step by step.

Thank you so much

#### retirede

##### Senior Member
Sizing of generator

You only have 3 loads totaling 68kw. Unless there is some provision to ensure they cannot all run concurrently, the generator must be sized to handle the entire load. Without doing some research, my guess is that a 75kw generator is probably the smallest standard size you can use.

ETA: I’m assuming the heaters are 19kw EACH.

#### topgone

##### Senior Member
Good day.

I would like to ask on how to size a generator set with given loads.

I have 2 heaters rated as 19 kW and a 30 kW motor. The genset will run 16 hours per day so the generator to be used should be Prime Power.

Appreciated if you can teach it step by step.

Thank you so much
My spreadsheet tells me you could use an 80 kVA genset. I assumed your motor is started DOL, efficiency =95%, starting PF=0.35, running PF = 0.95 and the heaters are 95% efficient. Your genset voltage dip will be about 30%. If you can't allow that big a dip during starting the motor, use a bigger set.

#### texie

##### Senior Member
In an application like this where a single motor is the bulk of the load I would tread carefully. Not all gensets of a given size are the same. There are a lot of variables among brands and even within brands in how they are designed and can have vastly different motor starting abilities. I would work with the vendor and their own software tools.

#### engrkira

##### Member
My spreadsheet tells me you could use an 80 kVA genset. I assumed your motor is started DOL, efficiency =95%, starting PF=0.35, running PF = 0.95 and the heaters are 95% efficient. Your genset voltage dip will be about 30%. If you can't allow that big a dip during starting the motor, use a bigger set.
Hi,

The 80 kVA genset will run as prime power not standby power? Can you tell me how you arrive with that rating? Summing the loads will result to 68 kW genset. How do you arrive with the 80 kVA capacity?

#### topgone

##### Senior Member
Hi,

The 80 kVA genset will run as prime power not standby power? Can you tell me how you arrive with that rating? Summing the loads will result to 68 kW genset. How do you arrive with the 80 kVA capacity?
1. First, you look at the loads your genset is going to supply power with.
2. Second, you have to separate the motor load and the static loads that you have.
3. Know your motor specs, you will know how much starting kVA is needed to start it and the kVA needed to run the motor.
4. The same thing is done to static loads, know how much kVA is needed when starting/ running them.
5. You will add the starting kVAs and the running kVAs and see choose a genset that will provide the necessary starting kVA as well as the running kVA. (If you are planning to start these equipment at the same time.)
6. Keep in mind that the figures you are dealing with are referred to the shaft output of the genset (not the electrical values-->use the efficiencies of each equipment.
7. Check if the voltage dip is not too much. This can be estimated by looking for the transient reactance of your genset (from the received documentation of the genset) and computing for the voltage dip, which is equal to VD = (X'd)/(X'd + (rated kVA of genset/starting kVA)). See MG1 32

If you can, do a staged starting-> you start every equipment in stages.

#### kwired

##### Electron manager
Hi,

The 80 kVA genset will run as prime power not standby power? Can you tell me how you arrive with that rating? Summing the loads will result to 68 kW genset. How do you arrive with the 80 kVA capacity?
Depends on how much voltage dip you can tolerate when the motor is starting, 80kVA may not be enough generator if you can't tolerate much dip at all.